Several types of 10BaseT Ethernet applications require a crossover or null cable. These applications include hubless connection of two Ethernet devices, connection of two hubs that lack an uplink port, or connection of a cable modem to a hub that lacks an uplink port.

A crossover cable is not expensive. They run $7-$12 retail at some discount electronic supply houses. If you have spare cable, connectors and a crimping tool, you can make one yourself. If you're like me, however, you have no tools and you're not happy about spending $7-$12 for something that costs a coupla bucks to manufacture. You want an alternative.

Another benefit of using a crossover coupler is that you can replace it with a hub in the future without changing out any of the cables. For example, if you have two computers connected to each other, and you later want to add a third computer or a printer, you can remove the crossover coupler, replace it with a hub, and run one more length of cable to the new device. If you had used a single crossover cable, you'd have to run three new lengths of cable to the location to the new hub.